Why isn't 'Loop selection' working if object only has edges?
For my understanding, the 4 edges of the top face of the cube are a loop.


Sure, they form a loop... but so do each of the other faces of the cube.

Each corner of the cube is a pole, where the edge flow can fork in either of three directions. So while it may be obvious to you which four edges you want to select, Blender has no way of knowing your intentions.